1. Field of the Invention
This invention relates to new and useful improvements in the processing of petroleum feed stocks and more particularly to a process for separation of fractions for different processes by absorption and desorption.
2. Brief Description of the Prior Art
It is well known that gasoline is composed of many different hydrocarbons having similar as well as quite different boiling points. It is also well known that different families or types of hydrocarbon molecules can have approximately the same boiling points, yet have significantly different octane numbers. The compounds having higher octane numbers perform better in high performance gasoline engines. Also in manufacturing gasoline, it is important that the blend of all the compounds in the gasoline blend have an effective octane which is sufficient for gasoline engines in which it will be used. Premium gasolines have higher octane numbers than do regular gasolines, and the premium gasoline may be used in higher performance engines.
Costs associated with present refinery processes for increasing the octane of motor gasolines result in premium grades costing significantly more than regular gasoline, even though their octane is only slightly higher than the regular grades. The phase-out of leaded gasoline for environmental reasons has contributed significantly to the costs of higher octane fuels. The future prospect of decreasing the amount of butanes in motor gasoline will further increase the costs of manufacturing higher octane gasolines. Limiting the aromatic content of motor gasoline because of air quality problems with high aromatic content fuels could limit the amount of premium gasoline available.
It is well known that light paraffinic hydrocarbons have higher octane numbers than heavier paraffinic hydrocarbons. Isoparaffins of the same molecular weight have higher octane numbers than normal paraffins. Likewise, highly branched paraffinic hydrocarbons have still higher octane numbers than isoparaffins of the same molecular weight.
Two current octane improvement processes based on these physical characteristics of hydrocarbons are namely: alkylation and normal pentane and normal hexane isomerization.
Alkylation, although it is the backbone for making high octane aviation gasoline, is expensive for motor gasoline grades. This process takes ethylene, propylene, butylene and pentylene compounds and reacts these materials with isobutane to form highly branched, high octane compounds. Slightly less than one and one-half barrels of feed are required to make one barrel of product. These compounds can be used in both aviation and motor gasolines.
Normal hexane and/or normal pentane are removed from a narrow cut hydrocarbon fraction by adsorption. These materials are isomerized to their respective isoparaffin. Any unisomerized normal paraffins may be recycled back through the adsorption cycle until all the normal compounds are converted to their respective isoparaffins. The isopentane produced has an octane number slightly higher than regular gasoline. The isohexanes produced have octane numbers almost 50 octane numbers above normal hexane, but still significantly lower than the octane of regular gasoline. High octane materials must be added to gasoline blend to make these isohexanes acceptable in motor gasoline.
All normal and iso- paraffins with molecular weight higher than normal hexane have such low octane numbers that they are more valuable as chemical and fluid catalytic cracking feed stock than as motor gasoline fractions. This severely limits the use of adsorption/isomerization because of the small amounts of normal pentane and normal hexane available for feed.
The most widely used process for upgrading virgin naphthas (55-70 octane) to motor gasoline is reforming. This process converts the naphthenes in virgin naphtha to higher octane aromatics. A slight loss of weight and significant loss in volume of gasoline occurs in this process. In spite of these losses, reforming is the best process now available for upgrading low octane naphtha.
Examples of two of the many possible naphthenes-to-aromatic reactions which occur in reforming are cyclohexane to benzene and isopropylcyclohexane to isopropylbenzene. Cyclohexane has octane numbers M+R/2=80.1 and the corresponding isopropylcyclohexane octane number is 62.0. The aromatics produced from both of these compounds exceed 100 octane. A portion of the isoparaffin and normal paraffins is converted to aromatics during the removal of hydrogen from the naphthenes. One of the more attractive aromatization reactions is the conversion of normal heptane (zero octane) to toluene (109 octane). The volume of gasoline loss per octane number gain for the stoichiometric conversion of cyclohexane, isopropylbenzene and normal heptane are: 0.56%/ON, 0.26%/ON, and 0.27%/ON respectively.
Some 20 to 30 percent of virgin naphthas are naphthenes. The octane numbers of these naphthas can be increased part of the way toward motor gasoline quality by the relatively efficient dehydrogenation step. In order to increase the octane numbers of the virgin naphthas to motor gasoline quality, the heavier normal paraffins and isoparaffins must be cracked to lighter, higher octane compounds or converted to aromatic compounds as a side reaction. A significant volume loss of gasoline blend stocks occurs in these cracking and/or aromatization steps. Unfortunately high octane branch paraffins are also cracked with a loss of high octane materials with little gain in octane.
Yields of motor gasoline equivalent octane raffinate stream from reformed naphthas are typically in the 80-85 volume percent. Frequently some of the high octane aromatics are extracted from the reformate for sales as more valuable chemical feed stocks. Reformate octane numbers may be increased above motor gasoline quality in order to offset this removal of aromatics and to blend lower octane materials into the motor gasoline pool. Frequently reformer operations are adjusted to produce reformate streams with octanes 5 to 10 octane numbers higher than motor gasoline. Under these severe conditions reformate yields of only 75 to 80 volume percent are realized.
The removal of normal paraffins from reformer feed by adsorption on molecular sieves for chemical feed stocks has been suggested in the literature. The quality of reformer feed would be improved and potentially valuable chemical feed stocks are produced. Yields of ethylene from cracking normal paraffins are reported to be 20-30 percent greater than from straight naphthas. The amount of total normal paraffins in reformer feed is usually only 5-15%. Unless the extracted normal paraffins are needed as a feed stock to a particular process, the costs of processing large quantities of feed for such low yields have been prohibitive.
The amounts of isoparaffins in reformer feed are some three to four times greater than amounts of normal paraffins. Although their octane numbers are slightly higher than their respective normal compound only isopentane and possibly isohexane are desirable components of motor gasoline.
As noted above, the use of adsorption/desorption processes is well known in petroleum processing and refining, especially for the separation of close boiling fractions which are otherwise difficult to separate. A number of U.S. patents are illustrative of the state of the art of adsorptive/desorptive separation.
Wylie U. S. Pat. No. 4,381,419 discloses an adsorption/desorption reparation process with integrated light and heavy desorbents.
Evans U.S. Pat. No. 4,804,802 discloses an isomerization process with adsorption/desorption separation.
Harandi U.S. Pat. No. 4,835,329 discloses the use of zeolite based catalysts in alkylation processes and absorptive separation.
Findley U.S. Pat. No. 4,874,524 discloses the separation of adsorbed components by variable temperature desorption.
Sircar U.S. Pat. No. 4,705,541 discloses production of mixed gases of controlled composition by pressure swing adsorption.
Pirkle U.S. Pat. No. 4,655,796 discloses continuous sorption process with simulated countercurrent flow.
Antrim U.S. Pat. No. 4,610,965 discloses adsorption-desorption purification of glucose isomerase; treatment with salt solutions.
Juntgen U.S. Pat. No. 4,432,774 discloses adsorption-desorption process for the recovery of hydrogen; pressurization-depressurization cycles.
Miwa U.S. Pat. No. 4,070,164 discloses adsorption-desorption pressure swing gas separation.
Geissler U.S. Pat. No. 4,029,717 discloses a simulated moving bed absorption-desorption process for paraxylene recovery.
Woodle U.S. Pat. No. 3,810,950 discloses a process for converting reactant into product including adsorption-desorption cycle for recycle of unreacted reactant.
Woodle U.S. Pat. No. 3,767,563 discloses an adsorption-desorption process for removing an unwanted component from a reaction charge mixture.
Reighter U.S. Pat. No. 3,675,392 discloses an adsorption-desorption method for purifying SF.sub.6.